Systems and methods for improving ultrasound image quality by applying weighting factors

ABSTRACT

Systems and methods for improving the quality of ultrasound images made up of a combination of multiple sub-images include giving more weight to sub-image information that is more likely to improve a combined image quality. Weighting factor information may be determined from the geometry (e.g., angle or path length) of a location of one or more specific transducer elements relative to a specific point within a region of interest or a region of an image. In some embodiments, any given pixel (or other discrete region of an image) may be formed by combining received echo data in a manner that gives more weight to data that is likely to improve image quality, and/or discounting or ignoring data that is likely to detract from image quality (e.g., by introducing noise or by increasing point spread).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/495,591, filed Apr. 24, 2017, now U.S. Pat. No. 11,172,911, which isa continuation of U.S. patent application Ser. No. 13/850,823, filedMar. 26, 2013, now U.S. Pat. No. 9,668,714, which claims the benefit ofU.S. Provisional Application No. 61/615,735, filed Mar. 26, 2012, titled“Systems and Methods for Improving Ultrasound Image Quality by ApplyingWeighting Factors”, all of which are incorporated by reference in theirentirety.

This application is related to U.S. Pat. No. 8,007,439, issued Aug. 30,2011 and titled “Method and Apparatus to Produce Ultrasonic Images UsingMultiple Apertures;” U.S. patent application Ser. No. 13/029,907,published as 2011/0201933, now U.S. Pat. No. 9,146,313, and titled“Point Source Transmission and Speed-Of-Sound Correction UsingMultiple-Aperture Ultrasound Imaging;” U.S. patent application Ser. No.12/760,375, filed Apr. 14, 2010, published as 2010/0262013 and titled“Universal Multiple Aperture Medical Ultrasound Probe;” and U.S. patentapplication Ser. No. 13/279,110, filed Oct. 21, 2011, published as2012/0057428, now U.S. Pat. No. 9,282,945, and titled “Calibration ofUltrasound Probes;” all of which are incorporated herein by reference.

INCORPORATION BY REFERENCE

Unless otherwise specified herein, all patents, publications and patentapplications mentioned in this specification are herein incorporated byreference to the same extent as if each individual publication or patentapplication was specifically and individually indicated to beincorporated by reference.

FIELD

This invention generally relates to ultrasound imaging and moreparticularly to systems and methods for improving ultrasound imagingquality by applying weighting factors.

BACKGROUND

In conventional ultrasonic imaging, a focused beam of ultrasound energyis transmitted into body tissues to be examined and the returned echoesare detected and plotted to form an image. While ultrasound has beenused extensively for diagnostic purposes, conventional ultrasound hasbeen greatly limited by depth of scanning, speckle noise, poor lateralresolution, obscured tissues and other such problems.

In order to insonify body tissues, an ultrasound beam is typicallyformed and focused either by a phased array or a shaped transducer.Phased array ultrasound is a commonly used method of steering andfocusing a narrow ultrasound beam for forming images in medicalultrasonography. A phased array probe has many small ultrasonictransducer elements, each of which can be pulsed individually. Byvarying the timing of ultrasound pulses (e.g., by pulsing elements oneby one in sequence along a row), a pattern of constructive interferenceis set up that results in a beam directed at a chosen angle. This isknown as beam steering. Such a steered ultrasound beam may then be sweptthrough the tissue or object being examined. Data from multiple beamsare then combined to make a visual image showing a slice through theobject.

Traditionally, the same transducer or array used for transmitting anultrasound beam is used to detect the returning echoes. This designconfiguration lies at the heart of one of the most significantlimitations in the use of ultrasonic imaging for medical purposes: poorlateral resolution. Theoretically, the lateral resolution could beimproved by increasing the width of the aperture of an ultrasonic probe,but practical problems involved with aperture size increase have keptapertures small. Unquestionably, ultrasonic imaging has been very usefuleven with this limitation, but it could be more effective with betterresolution.

Significant improvements have been made in the field of ultrasoundimaging with the creation of multiple aperture imaging, examples ofwhich are shown and described in Applicant's prior patents andapplications referenced above. Multiple aperture imaging methods andsystems allow for ultrasound signals to be both transmitted and receivedfrom separate apertures.

SUMMARY OF THE DISCLOSURE

A method of forming an ultrasound image is provided, the methodcomprising transmitting an unfocused first circular wave frontultrasound signal into a region of interest from a first transmitaperture and receiving echoes of the first circular wave frontultrasound signal at a first receive aperture to form a first imagelayer, transmitting an unfocused second circular wave front ultrasoundsignal into a region of interest from a second transmit aperture andreceiving echoes of the second circular wave front ultrasound signal atthe first receive aperture to form a second image layer, applying aweighting factor to at least one pixel of the first image layer toobtain a modified first image layer, and combining the modified firstimage layer with the second image layer to form a combined image.

In some embodiments, the method further comprises applying a weightingfactor to a least one pixel of the second image layer to obtain amodified second image layer.

In other embodiments, the method further comprises, prior to applyingthe weighting factor, determining a value of the weighting factor bydetermining an angle between a point represented by the at least onepixel and the first transmit aperture, and determining the value ofweighting factor as a mathematical function of the determined angle.

In one embodiment, the method further comprises, prior to applying theweighting factor, determining a value of the weighting factor bydetermining an angle between a point represented by the at least onepixel and the first receive aperture, and determining the weightingfactor as a mathematical function of the determined angle.

In some embodiments, determining the value of the weighting factorcomprises determining whether the angle exceeds a threshold value,selecting a first value for the weighting factor if the angle exceedsthe threshold value, and selecting a second value for the weightingfactor if the angle does not exceed the threshold value.

In other embodiments, determining the value of the weighting factorcomprises determining whether the angle exceeds a threshold value,selecting a first value for the weighting factor if the angle exceedsthe threshold value, and selecting a second value for the weightingfactor if the angle does not exceed the threshold value.

In one embodiment, the method further comprises, prior to applying theweighting factor, determining a value of the weighting factor bydetermining a first distance from one of the first or second transmitapertures to a point represented by the at least one pixel, determininga second distance from the point to the first receive aperture, summingthe first distance and the second distance to obtain a total pathlength, and determining the weighting factor as a mathematical functionof the total path length.

In some embodiments, applying the weighting factor comprises multiplyingthe weighting factor by a pixel intensity value of the at least onepixel.

In other embodiments, applying the weighting factor decreases the valueof pixels that are identified as likely to contain more than a thresholdlevel of noise.

In one embodiment, the method further comprises transmitting the firstcircular wave front at a first frequency, and transmitting the secondcircular wavefront at a second frequency, the first frequency beinggreater than the second frequency, and applying a weighting factor to atleast one pixel in the second image based on the difference between thefirst frequency and the second frequency.

In some embodiments, the mathematical function is selected from thegroup consisting of a monotonic function, a linear function, a normaldistribution, a parabolic function, a geometric function, an exponentialfunction, a Gaussian distribution, and a Kaiser-Bessel distribution.

In another embodiment, the method comprises, prior to applying theweighting factor, determining a value of the weighting factor byevaluating a quality of a point-spread-function of the first transmitaperture and the first receive aperture, determining that a pixel imageobtained using the first transmit aperture and the first receiveaperture will improve image quality, and assigning a non-zero positivevalue to the weighting factor.

In some embodiments, the method also comprises, prior to applying theweighting factor, determining a value of the weighting factor byevaluating a quality of a point-spread-function of the first transmitaperture and the first receive aperture, determining that a pixel imageobtained using the first transmit aperture and the first receiveaperture will degrade image quality, and assigning a value of zero tothe weighting factor.

In another embodiment, the method further comprises changing an imagewindow by zooming or panning to a different portion of the region ofinterest, determining a new weighting factor value based on the changedimage window.

A method of identifying transmit elements not blocked by an obstacle isalso provided, the method comprising transmitting an unfocused firstcircular wave front ultrasound signal from a first transmit aperture andreceiving echoes of the first circular wave front ultrasound signal at afirst receive aperture, determining whether deep echo returns fromwithin the region of interest are received by identifying if a timedelay associated with the received echoes exceeds a threshold value, andidentifying the first transmit aperture as being clear of an obstacle ifdeep echo returns are received.

Another method of identifying transducer elements blocked by an obstacleis provided, the method comprising transmitting an unfocused firstcircular wave front ultrasound signal from a first transmit aperture andreceiving echoes of the first circular wave front ultrasound signal at afirst receive aperture, determining whether strong shallow echo returnsare received by identifying a plurality of echo returns with intensityvalues greater than a threshold intensity and with time delays less thana threshold time delay, and identifying the first transmit aperture asbeing blocked by an obstacle if strong shallow echo returns arereceived.

An ultrasound imaging system is also provided comprising an ultrasoundtransmitter configured to transmit unfocused ultrasound signals into aregion of interest, an ultrasound receiver configured to receiveultrasound echo signals returned by reflectors in the region ofinterest, a beamforming module configured to determine positions of thereflectors within the region of interest for displaying images of thereflectors on a display, first user-adjustable controls configured toselect a designated aperture from a plurality of transmit apertures andreceive apertures of the ultrasound transmitter and ultrasound receiver,and second user-adjustable controls configured to increase or decrease aspeed-of-sound value used by the beamforming module to determine thepositions of reflectors detected with the designated aperture.

In one embodiment, the designated aperture is a transmit aperture. Inanother embodiment, the designated aperture is a receive aperture.

Another ultrasound imaging system is provided, comprising a firsttransmit aperture configured to transmit first and second unfocusedcircular wave front ultrasound signals into a region of interest, afirst receive aperture configured to receive echoes of the first andsecond circular wave front ultrasound signals, and a controllerconfigured to form a first image layer from received echoes of the firstcircular wave front ultrasound signal, and configured to form a secondimage layer from received echoes of the second circular wave frontultrasound signal, the controller being further configured to apply aweighting factor to at least one pixel of the first image layer toobtain a modified first image layer, and to combine the modified firstimage layer with the second image layer to form a combined image.

In some embodiments, the controller is configured to apply a weightingfactor to a least one pixel of the second image layer to obtain amodified second image layer.

In other embodiments, the controller is configured to determine a valueof the weighting factor by determining an angle between a pointrepresented by the at least one pixel and the first transmit aperture,the controller being further configured to determine the value ofweighting factor as a mathematical function of the determined angle.

In some embodiments, the controller is configured to determine a valueof the weighting factor by determining an angle between a pointrepresented by the at least one pixel and the first receive aperture,the controller being further configured to determine the weightingfactor as a mathematical function of the determined angle.

In one embodiment, determining the value of the weighting factorcomprises determining whether the angle exceeds a threshold value,selecting a first value for the weighting factor if the angle exceedsthe threshold value, and selecting a second value for the weightingfactor if the angle does not exceed the threshold value.

In another embodiment, determining the value of the weighting factorcomprises determining whether the angle exceeds a threshold value,selecting a first value for the weighting factor if the angle exceedsthe threshold value, and selecting a second value for the weightingfactor if the angle does not exceed the threshold value.

In some embodiments, the controller is configured to determine a valueof the weighting factor by determining a first distance from one of thefirst or second transmit apertures to a point represented by the atleast one pixel, determining a second distance from the point to thefirst receive aperture, summing the first distance and the seconddistance to obtain a total path length, and determining the weightingfactor as a mathematical function of the total path length.

In one embodiment, applying the weighting factor comprises multiplyingthe weighting factor by a pixel intensity value of the at least onepixel.

In another embodiment, applying the weighting factor decreases the valueof pixels that are identified as likely to contain more than a thresholdlevel of noise.

In some embodiments, the first transmit aperture is configured totransmit the first circular wave front at a first frequency and thesecond circular wavefront at a second frequency, the first frequencybeing greater than the second frequency, and the controller isconfigured to apply a weighting factor to at least one pixel in thesecond image based on the difference between the first frequency and thesecond frequency.

In another embodiment, the mathematical function is selected from thegroup consisting of a monotonic function, a linear function, a normaldistribution, a parabolic function, a geometric function, an exponentialfunction, a Gaussian distribution, and a Kaiser-Bessel distribution.

In another embodiment, the controller, prior to applying the weightingfactor, is configured to determine a value of the weighting factor byevaluating a quality of a point-spread-function of the first transmitaperture and the first receive aperture, the controller being configuredto determine that a pixel image obtained using the first transmitaperture and the first receive aperture will improve image quality, thecontroller being further configured to assign a non-zero positive valueto the weighting factor.

In some embodiments, the controller is configured to determine a valueof the weighting factor by evaluating a quality of apoint-spread-function of the first transmit aperture and the firstreceive aperture, the controller being configured to determine that apixel image obtained using the first transmit aperture and the firstreceive aperture will degrade image quality, the controller beingfurther configured to assign a value of zero to the weighting factor.

In another embodiment, the controller is further configured to change animage window by zooming or panning to a different portion of the regionof interest, the controller further being configured to determine a newweighting factor value based on the changed image window.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1A is a perspective view illustration of an embodiment of amultiple aperture ultrasound imaging control system.

FIG. 1B is a block diagram illustrating an embodiment of some componentsof an imaging system that may be used in combination with the systemsand methods herein.

FIG. 2 is a schematic illustration of a multiple aperture ultrasoundimaging probe with two transducer arrays and a grid of points/pixels tobe imaged.

FIG. 3 is a schematic illustration of a multiple aperture ultrasoundimaging probe with three transducer arrays and a grid of points/pixelsto be imaged.

FIG. 4 is a graph illustrating several embodiments of transfer functionsthat may be used to determine weighting factors based on a total pathlength from a transmit aperture to a reflector back to a receiveaperture.

FIG. 5 is a cross-sectional view of an ultrasound transducer arrayillustrating an effective angle of the transducer elements.

FIG. 6 is a schematic illustration of the probe of FIG. 2 showing twoexample transmit angles for a selected point and selected transmitapertures.

FIG. 7 is a schematic illustration of the probe of FIG. 2 showing twoexample receive angles for a selected point and selected receiveapertures.

FIG. 8 is a graph illustrating several embodiments of transfer functionsthat may be used to determine weighting factors based on an aperturetransmit angle and/or an aperture receive angle.

FIG. 9 is a schematic illustration of a multiple aperture ultrasoundimaging probe with three transducer arrays and a grid of points/pixelsto be imaged, with an obstacle between the probe and the image field.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. References made to particular examples andimplementations are for illustrative purposes, and are not intended tolimit the scope of the invention or the claims.

The present disclosure provides systems and methods for improving thequality of ultrasound images made up of a combination of multiplesub-images by assigning relatively more weight to sub-image informationthat is more likely to improve overall quality of a combined image. Insome cases, this may be achieved by amplifying the effect of higherquality sub-image information. In other embodiments, image optimizationmay be achieved by reducing the effect of lower quality sub-imageinformation. In some embodiments, such information may be determinedfrom a known location of a specific transducer element relative to aspecific region of an image. In some embodiments, any given pixel (orother discrete region of an image) may be formed by combining receivedecho data in a manner that gives more weight to data that is likely toimprove image quality, and/or discounting or ignoring data that islikely to detract from image quality (e.g., by introducing noise or byincreasing point spread). Details of systems and methods for achievingsuch improvements are provided herein.

Although the various embodiments are described herein with reference toultrasound imaging of various anatomic structures, it will be understoodthat many of the methods and devices shown and described herein may alsobe used in other applications, such as imaging and evaluatingnon-anatomic structures and objects.

A multiple aperture ultrasound system may include a control unitcontaining electronics, hardware, software, and user interfacecomponents for controlling a multiple aperture imaging process. FIG. 1Aillustrates an example of a multiple aperture ultrasound imaging controlsystem 100 which has a control panel 120 and a display screen 130. Theimaging control system also contains electronic hardware and softwareconfigured to transmit, receive and process ultrasound signals using amultiple aperture ultrasound imaging (MAUI) probe. Such hardware andsoftware is generically referred to herein as MAUI electronics. In someembodiments, a MAUI control system may also include a calibration unit(not shown). In such embodiments, a calibration unit may beelectronically connected to the MAUI electronics by any wired orwireless communications system. In further embodiments, the electronicscontrolling a calibration system, including electronics controlling aprobe during calibration, may be entirely independent (physically and/orelectronically) of the electronics used for controlling an ultrasoundimaging process. Some examples of suitable calibration systems are shownand described in U.S. application Ser. No. 13/279,110 (Pub. No.2012/0057428), which is incorporated herein by reference. In someembodiments, the MAUI electronics may include only hardware and softwaresufficient to perform a portion of an imaging process. For example, insome embodiments the system 100 may include only controls andelectronics for capturing image data, while hardware, software,electronics and controls for processing and displaying an image may beexternal to the system 100.

As used herein the terms “ultrasound transducer” and “transducer” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies, and may refer without limitation toany single component capable of converting an electrical signal into anultrasonic signal and/or vice versa. For example, in some embodiments,an ultrasound transducer may comprise a piezoelectric device. In otherembodiments, ultrasound transducers may comprise capacitive micromachined ultrasound transducers (CMUT) or any other transducing devicecapable of converting ultrasound waves to and from electrical signals.

Transducers are often configured in arrays of multiple individualtransducer elements. As used herein, the terms “transducer array” or“array” generally refers to a collection of transducer elements mountedto a common backing plate. Such arrays may have one dimension (1D), twodimensions (2D), 1.X dimensions (1.XD) or three dimensions (3D). Otherdimensioned arrays as understood by those skilled in the art may also beused. Annular arrays, such as concentric circular arrays and ellipticalarrays may also be used. An element of a transducer array may be thesmallest discretely functional component of an array. For example, inthe case of an array of piezoelectric transducer elements, each elementmay be a single piezoelectric crystal or a single machined section of apiezoelectric crystal.

As used herein, the terms “transmit element” and “receive element” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies. The term “transmit element” mayrefer without limitation to an ultrasound transducer element which atleast momentarily performs a transmit function in which an electricalsignal is converted into an ultrasound signal. Transmitted ultrasoundsignals may be focused in a particular direction, or may be unfocused,transmitting in all directions or a wide range of directions. Similarly,the term “receive element” may refer without limitation to an ultrasoundtransducer element which at least momentarily performs a receivefunction in which an ultrasound signal impinging on the element isconverted into an electrical signal. Transmission of ultrasound into amedium may also be referred to herein as “insonifying.” An object orstructure which reflects ultrasound waves may be referred to as a“reflector” or a “scatterer.”

As used herein, the term “aperture” may refer to a conceptual “opening”through which ultrasound signals may be sent and/or received. In actualpractice, an aperture is simply a single transducer element or a groupof transducer elements that are collectively managed as a common groupby imaging control electronics. For example, in some embodiments anaperture may be a grouping of elements which may be physically separateand distinct from elements of an adjacent aperture. However, adjacentapertures need not necessarily be physically separate or distinct.Conversely, a single aperture may include elements of two or morephysically separate or distinct transducer arrays. For example ordistinct groups of transducer elements (e.g., a “left aperture” may beconstructed from a left array, plus the left half of a physicallydistinct center array, while a “right aperture” may be constructed froma right array, plus the right half of a physically distinct centerarray).

It should be noted that the terms “receive aperture,” “insonifyingaperture,” and/or “transmit aperture” are used herein to mean anindividual element, a group of elements within an array, or even entirearrays, that perform the desired transmit or receive function from adesired physical viewpoint or aperture. In some embodiments, suchtransmit and receive apertures may be created as physically separatecomponents with dedicated functionality. In other embodiments, anynumber of send and/or receive apertures may be dynamically definedelectronically as needed. In other embodiments, a multiple apertureultrasound imaging system may use a combination of dedicated-functionand dynamic-function apertures.

As used herein, the term “total aperture” refers to the total cumulativesize of all imaging apertures. In other words, the term “total aperture”may refer to one or more dimensions defined by a maximum distancebetween the furthest-most transducer elements of any combination of sendand/or receive elements used for a particular imaging cycle. Thus, thetotal aperture is made up of any number of sub-apertures designated assend or receive apertures for a particular cycle. In the case of asingle-aperture imaging arrangement, the total aperture, sub-aperture,transmit aperture, and receive aperture may all have the samedimensions. In the case of a multiple aperture imaging arrangement, thedimensions of the total aperture includes the sum of the dimensions ofall send and receive apertures.

In some embodiments, two apertures may be located adjacent to oneanother on a continuous array. In still other embodiments, two aperturesmay overlap one another on a continuous array, such that at least oneelement functions as part of two separate apertures. The location,function, number of elements and physical size of an aperture may bedefined dynamically in any manner needed for a particular application.Constraints on these parameters for a particular application will bediscussed below and/or will be clear to the skilled artisan.

Elements and arrays described herein may also be multi-function. Thatis, the designation of transducer elements or arrays as transmitters inone instance does not preclude their immediate re-designation asreceivers in the next instance. Moreover, embodiments of the controlsystem herein include the capabilities for making such designationselectronically based on user inputs, pre-set scan or resolutioncriteria, or other automatically determined criteria.

As used herein the term “point source transmission” may refer to anintroduction of transmitted ultrasound energy into a medium from singlespatial location. This may be accomplished using a single ultrasoundtransducer element or combination of adjacent transducer elementstransmitting together as a single transmit aperture. A singletransmission from a point source transmit aperture approximates auniform spherical wave front, or in the case of imaging a 2D slice, auniform circular wave front within the 2D slice. In some cases, a singletransmission of a circular, semi-circular, spherical or semi-sphericalwave front from a point source transmit aperture may be referred toherein as an “unfocused circular wave front ultrasound signal”, a“ping”, or a “point source pulse.”

Point source transmission differs in its spatial characteristics from a“phased array transmission” which focuses energy in a particulardirection from the transducer element array. A point source pulse (ping)may be transmitted so as to generate a circular wavefront in thescanning plane. Images may then be reconstructed from echoes assumingthat the wavefronts emitted from point source transmitters arephysically circular in the region of interest. In actuality, thewavefront may also have some penetration in the dimension normal to thescanning plane (i.e., some energy may essentially “leak” into thedimension perpendicular to the desired two-dimensional scanning plane,reducing the effective imaging reach). Additionally, the “circular”wavefront may actually be limited to a semicircle or a fraction of acircle less than 180 degrees ahead of the front face of the transduceraccording to the unique off-axis properties of the transducing material.Phased array transmission, on the other hand, manipulates thetransmission phase of a group of transducer elements in sequence so asto strengthen or steer an insonifying wave to a specific region ofinterest (mathematically and physically, this is done by means ofconstructive and destructive interference along multiple overlappingwavefronts). Phased array transmission also suffers the samethird-dimension energy leakage (off plane) as point source transmission.A short duration phased array transmission may be referred to herein asa “phased array pulse.”

The block diagram of FIG. 1B illustrates components of an ultrasoundimaging system 200 that may be used in combination with variousembodiments of systems and methods as described herein. The system 200of FIG. 1B may include several subsystems: a transmit control subsystem204, a probe subsystem 202, a receive subsystem 210, an image generationsubsystem 230, and a video subsystem 240. In various embodiments, thesystem 200 may also include one or more memory devices for containingvarious data for use during one or more ultrasound imaging steps. Suchmemory devices may include a raw echo data memory 220, a weightingfactor memory 235, a calibration data memory 238, an image buffer 236and/or a video memory 246. In various embodiments all data (includingsoftware and/or firmware code for executing any other process) may bestored on a single memory device. Alternatively, separate memory devicesmay be used for one or more data types. Further, any of the modules orcomponents represented in FIG. 2B may be implemented using any suitablecombination of electronic hardware, firmware and/or software.

The transmission of ultrasound signals from elements of the probe 202may be controlled by a transmit control subsystem 204. In someembodiments, the transmit control subsystem 204 may include anycombination of analog and digital components for controlling transducerelements of the probe 202 to transmit un-focused ultrasound pings atdesired frequencies and intervals from selected transmit aperturesaccording to a desired imaging algorithm. In some embodiments a transmitcontrol system 204 may be configured to transmit ultrasound pings at arange of ultrasound frequencies. In some (though not all) embodiments,the transmit control subsystem may also be configured to control theprobe in a phased array mode, transmitting focused (i.e., transmitbeamformed) ultrasound scanline beams.

In some embodiments, a transmit control sub-system 204 may include atransmit signal definition module 206 and a transmit element controlmodule 208. The transmit signal definition module 206 may includesuitable combinations of hardware, firmware and/or software configuredto define desired characteristics of a signal to be transmitted by anultrasound probe. For example, the transmit signal definition module 206may establish (e.g., based on user inputs or on predetermined factors)characteristics of an ultrasound signal to be transmitted such as apulse start time, pulse length (duration), ultrasound frequency, pulsepower, pulse shape, pulse direction (if any), pulse amplitude, transmitaperture location, or any other characteristics.

The transmit element control module 208 may then take information aboutthe desired transmit pulse and determine appropriate electrical signalsto be sent to appropriate transducer elements in order to produce thedesignated ultrasound signal. In various embodiments, the signaldefinition module 206 and the transmit element control module 208 maycomprise separate electronic components, or may include portions of oneor more common components.

Upon receiving echoes of transmitted signals from a region of interest,the probe elements may generate time-varying electric signalscorresponding to the received ultrasound vibrations. Signalsrepresenting the received echoes may be output from the probe 202 andsent to a receive subsystem 210. In some embodiments, the receivesubsystem may include multiple channels, each of which may include ananalog front-end device (“AFE”) 212 and an analog-to-digital conversiondevice (ADC) 214. In some embodiments, each channel of the receivesubsystem 210 may also include digital filters and data conditioners(not shown) after the ADC 214. In some embodiments, analog filters priorto the ADC 214 may also be provided. The output of each ADC 214 may bedirected into a raw data memory device 220. In some embodiments, anindependent channel of the receive subsystem 210 may be provided foreach receive transducer element of the probe 202. In other embodiments,two or more transducer elements may share a common receive channel.

In some embodiments, an analog front-end device 212 (AFE) may performcertain filtering processes before passing the signal to ananalog-to-digital conversion device 214 (ADC). The ADC 214 may beconfigured to convert received analog signals into a series of digitaldata points at some predetermined sampling rate. Unlike most ultrasoundsystems, some embodiments of the ultrasound imaging system of FIG. 3 maythen store digital data representing the timing, phase, magnitude and/orthe frequency of ultrasound echo signals received by each individualreceive element in a raw data memory device 220 before performing anyfurther receive beamforming, filtering, image layer combining or otherimage processing.

In order to convert the captured digital samples into an image, the datamay be retrieved from the raw data memory 220 by an image generationsubsystem 230. As shown, the image generation subsystem 230 may includea beamforming block 232 and an image layer combining (“ILC”) block 234.In some embodiments, a beamformer 232 may be in communication with acalibration memory 238 that contains probe calibration data. Probecalibration data may include information about the precise position,operational quality, and/or other information about individual probetransducer elements. The calibration memory 238 may be physicallylocated within the probe, within the imaging system, or in locationexternal to both the probe and the imaging system.

In some embodiments, after passing through the image generation block230, image data may then be stored in an image buffer memory 236 whichmay store beamformed and (in some embodiments) layer-combined imageframes. A video processor 242 within a video subsystem 240 may thenretrieve image frames from the image buffer, and may process the imagesinto a video stream that may be displayed on a video display 244 and/orstored in a video memory 246 as a digital video clip, e.g. as referredto in the art as a “cine loop”.

In some embodiments, the AFE 212 may be configured to perform variousamplification and filtering processes to a received analog signal beforepassing the analog signal to an analog-to-digital conversion device. Forexample, an AFE 212 may include amplifiers such as a low noise amplifier(LNA), a variable gain amplifier (VGA), a bandpass filter, and/or otheramplification or filtering devices. In some embodiments, an AFE device212 may be configured to begin passing an analog signal to an ADC 214upon receiving a trigger signal. In other embodiments, an AFE device canbe “free running”, continuously passing an analog signal to an ADC.

In some embodiments, each analog-to-digital converter 214 may generallyinclude any device configured to sample a received analog signal at someconsistent, predetermined sampling rate. For example, in someembodiments, an analog-to-digital converter may be configured to recorddigital samples of a time-varying analog signal at 25 MHz, which is 25million samples per second or one sample every 40 nanoseconds. Thus,data sampled by an ADC may simply include a list of data points, each ofwhich may correspond to a signal value at a particular instant. In someembodiments, an ADC 214 may be configured to begin digitally sampling ananalog signal upon receiving a trigger signal. In other embodiments, anADC device can be “free running”, continuously sampling a receivedanalog signal.

In some embodiments, the raw data memory device 220 may include anysuitable volatile or non-volatile digital memory storage device. In someembodiments, the raw data memory 220 may also comprise communicationelectronics for transmitting raw digital ultrasound data to an externaldevice over a wired or wireless network. In such cases, the transmittedraw echo data may be stored on the external device in any desiredformat. In other embodiments, the raw data memory 220 may include acombination of volatile memory, non-volatile memory and communicationelectronics.

In some embodiments, the raw data memory device 220 may comprise atemporary (volatile or non-volatile) memory section, and a long-termnon-volatile memory section. In an example of such embodiments, thetemporary memory may act as a buffer between the ADC 214 and thebeamformer 232 in cases where the beamformer 232 may be unable tooperate fast enough to accommodate data at the full rate from the ADC214. In some embodiments, a long-term non-volatile memory device may beconfigured to receive data from a temporary memory device or directlyfrom the ADC 214. Such a long-term memory device may be configured tostore a quantity of raw echo data for subsequent processing, analysis ortransmission to an external device.

In some embodiments, the beamforming block 232 and the image layercombining block 234 may each include any digital signal processingand/or computing components configured to perform the specifiedprocesses (e.g., as described below). For example, in variousembodiments the beamforming 232 and image layer combining 234 may beperformed by software running on a single GPU, on multiple GPUs, on oneor more CPUs, on combinations of CPUs & GPUs, on single or multipleaccelerator cards or modules, on a distributed processing system, or aclustered processing system. Alternatively, these or other processes maybe performed by firmware running on an FPGA architecture or one or morededicated ASIC devices.

In some embodiments, the video processor 242 may include any videoprocessing hardware, firmware and software components that may beconfigured to assemble image frames into a video stream for displayand/or storage.

A weighting factor memory device may contain data defining weightingfactor values to be applied during beamforming, image layer combining,image processing, or any other stage of image formation as desired.Examples of various types of weighting factors are provided below.

FIG. 2 illustrates an embodiment of a multiple aperture ultrasoundimaging probe 10 and a region of interest 20 to be imaged represented asa grid. The probe 10 is shown with a left transducer array 12 whichincludes three transmit apertures labeled ‘n,’ ‘j,’ and ‘k’ (which maybe referred to herein by short-hand designations Ln, Lj and Lk). A righttransducer array 14 also includes three transmit apertures ‘n,’ ‘j,’ and‘k’ (which may be referred to herein by short-hand designations Rn, Rjand Rk). Some or all of the elements of the left transducer array 12 mayalso be designated as a left receive aperture 13. Similarly, some or allof the elements of the right transducer array 14 may be designated as aright receive aperture 15.

FIG. 3 illustrates an embodiment of a three-array multiple apertureultrasound imaging probe 11. In addition to the left and right arrays ofthe two-array probe, the three-array probe includes a center transducerarray 16, which includes three transmit apertures labeled ‘n,’ ‘j,’ and‘k’ (which may be referred to herein by short-hand designations Cn, Cjand Ck). Some or all of the elements of the center transducer array 16may also be designated as a center receive aperture 17. In otherembodiments, any other multiple aperture probe construction may also beused with the systems and methods described herein. For example,Applicant's prior applications describe several alternative probeconstructions, such as probes with four, five or more arrays, probeswith one or more adjustable arrays, probes with one or more large arraysthat may be electronically sub-divided into any number of apertures, andsingle-aperture probes, any of which (or others) may be used inconnection with the systems and methods described herein.

In some embodiments, the width of a receive aperture may be limited bythe assumption that the average speed of sound is approximately the samefor every path from a scatterer to each element of the receive aperture.Given a sufficiently narrow receive aperture this simplifying assumptionis acceptable. However, as the receive aperture width increases, atipping point is reached (referred to herein as the “maximum coherentaperture width” or “maximum coherence width”), beyond which the echoreturn paths will necessarily pass though different types of tissuehaving intrinsically different speeds of sound. When this differenceresults in receive wavefront phase shifts approaching or exceeding 180degrees, additional receive elements extended beyond the maximumcoherent receive aperture width will actually degrade the image ratherthan improve it.

Therefore, in order to realize the inherent benefits of a wide probewith a total aperture width far greater than the maximum coherentaperture width, the full probe width may be physically or logicallydivided into multiple apertures, each of which may be limited to aneffective width less than or equal to the maximum coherent aperturewidth, and thus small enough to avoid phase cancellation of receivedsignals. The maximum coherence width can be different for differentpatients and for different probe positions on the same patient. In someembodiments, a compromise width may be determined for a given probesystem. In other embodiments, a multiple aperture ultrasound imagingcontrol system may be configured with a dynamic algorithm to subdividethe available elements in multiple apertures into groups that are smallenough to avoid significant image-degrading phase cancellation.

In some embodiments, it may be difficult or impossible to meetadditional design constraints while grouping elements into apertureswith a width less than the maximum coherence width. For example, ifmaterial is too heterogeneous over very small areas, it may beimpractical to form apertures small enough to be less than the maximumcoherence width. Similarly, if a system is designed to image a verysmall target at a substantial depth, an aperture with a width greaterthan the maximum coherence width may be needed. In such cases, a receiveaperture with a width greater than the maximum coherence width can beaccommodated by making additional adjustments or corrections to accountfor differences in the speed-of-sound along different paths. Someexamples of such speed-of-sound adjustments are provided here, whileadditional examples are provided in US Patent Application Pub. No.2010/0201933, titled “Universal Multiple Aperture Medical UltrasoundProbe.” In some embodiments, because the maximum coherence width isultimately variable from patient-to-patient and fromlocation-to-location even for a single patient, it may be desirable toprovide a user-interface adjustment configured to allow a user toselectively increase or decrease a maximum coherence width during animaging session, or during post-processing of stored raw echo data. Theadjusted maximum coherence width may be applied by correspondinglyincreasing or decreasing the size of receive apertures (i.e., transducerelement groups) to be used during beamforming.

Image Layer Combining

In some embodiments, multiple aperture imaging using a series oftransmit pings may operate by transmitting a point-source ping from afirst transmit aperture and receiving echoes with elements of two ormore receive apertures. A complete image may be formed by triangulatingthe position of scatterers based on delay times between pingtransmission and echo reception. As a result, a complete image may beformed by a controller or control system from data received at eachreceive aperture from echoes of each transmit ping. Images obtained fromdifferent unique combinations of a ping and a receive aperture may bereferred to herein as image layers. Multiple image layers may becombined to improve the overall quality of a final combined image. Thus,in some embodiments the total number of image layers generated can bethe product of the number of receive apertures and the number oftransmit apertures (where a “transmit aperture” can be a single transmitelement or a group of transmit elements).

For example, in some embodiments, a single time domain frame may beformed by combining image layers formed from echoes at two or morereceive apertures from a single transmit ping. In other embodiments, asingle time domain frame may be formed by combining image layers formedfrom echoes at one or more receive apertures from two or more transmitpings. In some such embodiments, the multiple transmit pings mayoriginate from different transmit apertures.

For example, in one embodiment with reference to FIG. 2, a first imagelayer (representing all points in the grid 20, or only a section of thegrid 20 if panning/zooming to a particular target of interest 30) may beformed by transmitting a first ping from a first transmit aperture Lnand receiving echoes of the first ping at a left receive aperture 13. Asecond image layer may be formed from echoes of the first ping receivedat the right receive aperture 15. Third and fourth image layers may beformed by transmitting a second ping from a second transmit aperture Ljand receiving echoes of the second ping at the left receive aperture 13and the right receive aperture 15. In some embodiments, all four imagelayers may then be combined to form a single time domain image frame. Inother embodiments, a single time domain image frame may be obtained fromechoes received at any number of receive apertures from any number ofpings transmit by any number of transmit apertures. Time domain imageframes may then be displayed sequentially on a display screen as acontinuous moving image. Still images may also be formed by combiningimage layers using any of the above techniques.

Display screens and the images displayed on them may generally bedivided into a grid of pixels. In some cases, a pixel is the smallestindividually controllable area of a display screen. Relationshipsbetween image pixels and display pixels are generally well understood inthe art, and will not be described here. For the purposes of the presentdescription, the square cells of the grids 20 shown in the figures willbe referred to as pixels. In many of the embodiments herein, groups ofpixels may be treated together as a common group. Thus, the use of theterm “pixel” is not intended to be limited to any particular size, butis used as a convenient term for describing a discrete section of animage.

In a monochrome display each pixel is assigned only one value:“intensity”, which is a scalar value that defines how much light thepixel should display. In a color display, in addition to an intensityvalue, each pixel may be assigned multiple color component values, suchas red, green, and blue; or cyan, magenta, yellow, and black. Thefollowing description will primarily refer to applying weighting factorsagainst various contributions to a pixel's intensity from multiplesources. However, in some embodiments some or all color values may alsobe weighted using the same or related techniques.

With a multiple aperture probe using a point-source transmission imagingtechnique, each image pixel may be assembled by beamforming receivedecho data on a per-receive element basis, combining information fromechoes received at each of the multiple receive elements within each ofthe multiple receive apertures (the echoes having resulted from pingstransmitted from each of the multiple transmitters within each of themultiple transmit apertures). In some embodiments of multiple apertureimaging with point-source transmission, receive beamforming comprisesforming a pixel of a reconstructed image by summing time-delayed echoreturns on receive transducer elements from a scatterer in the objectbeing examined. The time delays of a scatterer's echoes recorded at eachreceiver are a function of the unique geometry of the probe elements,combined with an assumed value for the speed of sound through the mediumbeing imaged. An important consideration is whether the summation shouldbe coherent (phase sensitive) or incoherent (summing signal magnitudesonly and disregarding phase information). Further details of ping-basedbeamforming are described in Applicant's patent application Ser. No.13/029,907 which is incorporated herein by reference.

Summation or averaging of image layers resulting from multiple transmitpings may be accomplished either by coherent addition, incoherentaddition, or a combination of the two. Coherent addition (incorporatingboth phase and magnitude information during image layer summation) tendsto maximize lateral resolution, whereas incoherent addition (summingmagnitudes only and omitting phase information) tends to average outspeckle noise and minimize the effects of image layer alignment errorsthat may be caused by minor variations in the speed of sound through theimaged medium. Speckle noise is reduced through incoherent summingbecause each image layer will tend to develop its own independentspeckle pattern, and summing the patterns incoherently has the effect ofaveraging out these speckle patterns. Alternatively, if the patterns areadded coherently, they reinforce each other and only one strong specklepattern results. Incoherent addition may be thought of as akin toinstantaneous compound imaging, which has long been known as a means tosuppress speckle noise.

Variations in the speed of sound may be tolerated by incoherent additionas follows: Summing two pixels coherently with a speed-of-soundvariation resulting in only half a wavelength's delay (e.g.,approximately 0.25 mm for a 3 MHz probe) results in destructive phasecancellation, which causes significant image data loss; if the pixelsare added incoherently, the same or even greater delay causes only aninsignificant spatial distortion in the image layer and no loss of imagedata. The incoherent addition of such image layers may result in somesmoothing of the final image (in some embodiments, such smoothing may beadded intentionally to make the image more readable).

Image layer combining may be described in terms of three image layerlevels for which the determination of coherent vs. incoherent summingcan be made. These three cases include first-level image layers,second-level image layers and third-level image layers. (1) Afirst-level image layer may be formed from echoes received at a singlereceive aperture resulting from a single ping from a single transmitaperture. For a unique combination of a single ping and a single receiveaperture, the delayed echoes received by all the receive elements in thereceive aperture may be summed to obtain a first-level image layer. (2)Multiple first-level image layers resulting from echoes of multipletransmit pings (from the same or different transmit apertures) receivedat a single receive aperture can be summed together to produce asecond-level image layer. Second-level image layers may be furtherprocessed to improve alignment or other image characteristics. (3)Third-level images may be obtained by combining second-level imagelayers formed with data from multiple receive apertures. In someembodiments, third-level images may be displayed as sequentialtime-domain frames to form a moving image.

At all three image layer levels, coherent addition can lead to maximumlateral resolution of a multiple aperture system if the geometry of theprobe elements is known to a desired degree of precision and theassumption of a constant speed of sound across all paths is valid.Likewise, at all image layer levels, incoherent addition leads to thebest averaging out of speckle noise and tolerance of minor variations inspeed of sound through the imaged medium.

In some embodiments, coherent addition can be used to combine imagelayers resulting from apertures for which phase cancellation is notlikely to be a problem, and incoherent addition can then be used wherephase cancellation would be more likely to present a problem, such aswhen combining images formed from echoes received at different receiveapertures separated by a distance exceeding some threshold.

In some embodiments, all first-level images may be formed by usingcoherent addition assuming the receive apertures used were chosen tohave a width less than the maximum coherent aperture width. For secondand third level image layers, many combinations of coherent andincoherent summation are possible. For example, in some embodiments,second-level image layers may be formed by coherently summingcontributing first-level image layers, while third-level image layersmay be formed by incoherent summing of the contributing second-levelimage layers.

Speckle Preset Control

In other embodiments, it may be desirable to combine image layersthrough any of a wide variety of algorithms using combinations ofcoherent and incoherent summation. In some embodiments, an imagingcontrol system may be configured to store a plurality of selectablepre-programmed summation algorithms that may be designed for specificimaging applications. In some embodiments, stored summation algorithmsmay be manually selectable such as by operating a manual user interfacecontrol. Alternatively, stored summation algorithms may be automaticallyselectable based on other data or information available to the controlsystem.

For example, in some embodiments an alternative algorithm may compriseforming all second-level and third-level image layers by coherentaddition. In another embodiment, all second-level and/or third-levelimage layers may be formed by incoherent addition. In furtherembodiments, only selected combinations of second-level images may becombined coherently or incoherently to form third-level images. In otherembodiments, only selected combinations of first-level image layers maybe combined coherently to form second-level image layers.

In some embodiments, a first-level image layer may also be formed bysumming in-phase and quadrature echo data (i.e., summing each echo withan echo ¼ wavelength delayed) for each receive-aperture element. In mostembodiments, echoes received by elements of a single receive apertureare typically combined coherently. In some embodiments, the number ofreceive apertures and/or the size of each receive aperture may bechanged in order to maximize some desired combination of image qualitymetrics such as lateral resolution, speed-of-sound variation tolerance,speckle noise reduction, etc. In some embodiments, such alternativearrangements may be selectable by a user. In other embodiments, sucharrangements may be automatically selected or developed by an imagingsystem.

Once an image layer is formed by incoherent summation, any phaseinformation for that image layer is lost. Thus, any subsequent imagelayers using an image layer formed by incoherent summation willthemselves necessarily be incoherently combined. Thus, in someembodiments, phase information may be retained for as long as desired inan image-layer combining process.

Speed of Sound Control

As discussed above, a speed-of-sound value is typically assumed duringbeamforming in order to determine the location of ROI points andcorresponding pixels based on time delays between a transmit time and areceive time. In soft human tissue, the speed of sound is typicallyassumed to be about 1540 m/s. However, the speed of sound is known tovary by as much as 10% or more among patients and among different typesof soft tissue within a single patient. Variation between an assumedspeed-of-sound and an actual value for a particular scatterer path maycause errors during beamforming, which may cause a blurring or spatialdisplacement effect in an image. Therefore, in some embodiments amultiple aperture ultrasound imaging system may be configured to allowfor automatic and/or manual adjustment of an assumed speed of soundvalue for some or all scatterer paths.

In some embodiments, a multiple aperture imaging system may include a“coarse” speed-of-sound adjustment that increases or decreases anassumed value of speed-of-sound used in beamforming for all scattererpaths (i.e., for all combinations of transmit aperture and receiveaperture). In some cases, such an adjustment may also be provided forsingle-aperture ultrasound imaging systems. A coarse speed-of-soundadjustment may be manual (e.g., a dial, slider or any other physical orvirtual user interface device) to allow a sonographer or other user todirectly increase or decrease an assumed speed-of-sound value until thesystem produces a result acceptable to the user. In other embodiments, a“coarse” speed of sound adjustment may be controlled automatically by animaging control system. Thus, a coarse speed-of-sound adjustment mayapply a single adjustment to all image layers.

Various embodiments of “fine” speed-of-sound adjustments may also beprovided. In some embodiments, a fine speed-of-sound adjustment may beconfigured to adjust an assumed speed of sound value for a singlereceive aperture. In other embodiments, a fine speed-of-sound adjustmentmay be configured to adjust an assumed speed of sound value for a singletransmit aperture. In further embodiments, a fine speed-of-soundadjustment may be configured to adjust an assumed speed of sound valuefor one or more specific combinations of transmit aperture and receiveaperture. Thus, fine speed-of-sound controls may be configured toeffectively apply adjustments to specific first-level or second-levelimage layers. As with coarse speed-of-sound adjustments, finespeed-of-sound adjustments may be manual, automatic or a combination ofthe two.

In some embodiments, a coarse speed-of-sound adjustment may be mademanually by a user, and fine speed-of-sound adjustments may be madeautomatically by the ultrasound imaging control system. In otherembodiments, both coarse and fine speed-of-sound adjustments may beautomatically controlled. In some embodiments, the ultrasound imagingcontrol system may be configured to try out different coarse and/or finespeed of sound values until a desired image quality metric (e.g.,sharpness of edges or points, maximum contrast, maximum dynamic range,etc.) of the resulting image (or images) exceeds a threshold value.Alternatively any other “autofocus” algorithms may be applied to adjusta speed-of-sound value until an image quality metric is improved oroptimized.

In some cases, each unique pair of transmit aperture and receiveaperture may be referred to herein as a “view.” In other cases, a viewmay also refer to a unique combination of a single transmit transducerelement and a single receive transducer element. In embodiments in whichreceive apertures comprise a plurality of transducer elements, thegroups of receive elements may be treated collectively for the purposesof the following descriptions. Alternatively, even when part of areceive aperture group, individual receive elements may be treatedindividually in some embodiments. For example, if a multiple apertureimaging system utilizes 30 transmit apertures and three receiveapertures, each image pixel is potentially formed by the combination ofimage data from 90 different views. Alternately, treating each view as acombination of an individual transmit element and an individual receiveelement, and considering a probe with 48 transmit elements and 144receive elements, each pixel may potentially be formed by thecombination of image data from 6,912 distinct views. Images obtainedfrom such views may be aggregated through image layer combinations(e.g., as described above) to produce a smaller number of images orimage frames.

Unless otherwise specified, the grid 20 of FIGS. 2, 3, 6, 7 and 9simultaneously represents a grid of display pixels and a grid ofcorresponding points within a region of interest (“ROI”) in an objectbeing imaged. The term “ROI points” will be used herein to describepoints within the scan plane (or 3D scan volume) at fixed locationsrelative to the probe. As will become clear from the followingdescription, ROI points will not necessarily always correlate directlyto pixel locations. For example, if an image is “zoomed in” to representa smaller area 30, the grid of display pixels 20 would correspond onlyto the points within the zoomed area 30 in the region of interest.However, at any zoom level, the physical location of an ROI pointrepresented by a given image pixel may be determined (relative to theprobe) with a high degree of accuracy.

In some embodiments, a speed-of-sound value used during beamforming maybe based on a calculation of an average speed-of-sound through a numberof different materials, each material having a known averagespeed-of-sound. For example, when imaging a human patient, ultrasoundwaves may pass through and reflect from multiple different tissue types.Each type of tissue typically has a slightly different fundamentalspeed-of-sound. By identifying approximate dimensions of all of thetissues through which a given sound wave passes between a transmittransducer element and a reflector and between the reflector and areceive transducer element, an average speed-of-sound may be calculatedfor the complete sound wave path. In some embodiments, a weightedaverage may be used in which each material-specific speed-of-sound valueis weighted by a weighting factor that is proportional to a thickness ofthe material in the image plane. In some embodiments, performing such acalculation may provide a more accurate average speed-of-sound value foruse during a beamforming process which may improve the qualitybeamforming results relative to results obtained using a generic averagespeed-of-sound value.

In some embodiments, computer-automated-detection techniques (e.g.,various heuristic models) may be used to automatically identify one ormore tissue types within a patient, based on information such as ashape, position, reflectiveness, or other characteristics of thetissue(s). Alternatively, a user may identify tissues based on his orher own expertise and through the use of a suitable user interface(e.g., by circumscribing an organ in an ultrasound image obtained bybeamforming using an assumed speed-of-sound value).

In other embodiments, such techniques may be used in non-medical imagingcontexts, such as industrial non-destructive testing. In suchembodiments, the dimensions, structures and materials of an object to beimaged may be substantially known. Thus, average speed-of-sound valuesmay be calculated based on a known structure of the object and a knownposition of the transducer relative to the object.

Introducing Weighting Factors

In any of the various embodiments described herein, weighting factorsmay be applied at any appropriate point during the process of imageformation, from the receiving of analog echo signals through image layercombining to produce final image frames. For example, in someembodiments, some weighting may be applied to signals received from oneor more transducer elements when analog echo signals are received by anAFE (212 in FIG. 1B), during analog-to-digital conversion of echosignals by an A/D converter (214 in FIG. 1B), during beamformingperformed by a beamforming module (232 in FIG. 1B), or during imagelayer combining as performed by an image layer combining module (234 inFIG. 1B).

In some embodiments, weighting factors may be applied during beamformingby multiplying individual pixel values by a corresponding weightingfactor as each pixel is formed from received echoes. Alternatively, asingle weighting factor may be applied to all pixels in an entire imageduring beamforming, such as when a weighting factor is to be applied toall pixels involving an identified transmit or receive transducerelement. Applying weighting factors during beamforming means that a mostbasic image layer may be improved using weighting factors. In somecases, applying weighting factors during beamforming may be morecomputationally intensive than applying them later in an image layercombining process, but such low-level image layers may also retain moreoriginal information. In some embodiments, computational intensity mayneed to be balanced against image quality in order to maximize a resultusing a particular system.

Prior to combining image layers at any of the three levels describedabove, any individual image layer may be adjusted by applying one ormore weighting masks to increase or decrease the contribution of theentire image layer or only a portion of the image layer to a finalcombined image. In some embodiments, after applying a weighting maskand/or after combining image layers, a normalizing step may be appliedin order to cause all regions of a final image (e.g., a third-levelimage) to have a consistent average intensity.

For any given ROI point and corresponding pixel, some views will providehigher quality image data while other views may contribute lower qualitydata to the pixel. In some embodiments, one or more weighting factorsmay be used to increase the effect of high quality views on a displayedpixel and/or to diminish the effect of low quality views on a displayedpixel. For example, during image processing, the intensity magnitude ofany given pixel I_(p) may be obtained by multiplying the pixel intensityvalues from each contributing image layer by one or more correspondingweighting factors, and then summing the products. For example:I_(p)=Σw*I_(v). Where w is a weighting factor and I_(v) is the intensityobtained by a particular view (v). Such individual weighting factors maybe combined into a mask to be applied to an entire image layer.

In some embodiments, weighting factors may be pre-calculated for a for agiven set of pixel-view combinations, ROI point-view combinations,combinations of transmit aperture and a pixel or ROI point, orcombinations of receive aperture (or receive element) and a pixel or ROIpoint. Such pre-calculated weighting factors may be stored for laterretrieval, such as by a table lookup operation during imaging. In otherembodiments, weighting factors may be calculated or otherwise determined“on-the-fly” during imaging. In some embodiments, a different weightingfactor may be obtained for each unique pixel-view pair (and/or ROIpoint-view pair). In some embodiments, the quality of imaging dataprovided by a view with respect to a given pixel/ROI point may beevaluated in terms of a plurality of factors, two examples of whichinclude signal-to-noise (S/N) ratio and point spread function (PSF).

Weighting Factors Based on S/N Ratio

As used herein, the S/N ratio is a function of attenuation of anultrasound signal as it passes through an imaged medium. Thus, in someembodiments, signal-to-noise ratio S/N may be predicted as a function ofpath length. Path length refers to a total distance from a transmitaperture, to an ROI point and back to a receive aperture. In general, anultrasound signal will attenuate at a somewhat constant rate for eachunit of length traveled through a medium. Attenuation rates are wellunderstood by those skilled in the art, and may be a function of animaging medium, ultrasound frequency, the angle between the signal pathand the surfaces of both the transmit and receive elements, and otherfactors.

Therefore, all else being equal, echoes from objects close to an elementwill tend to be stronger and have better signal-to-noise ratios thanechoes from objects that are far away from an element. Thus, in someembodiments, a total distance between transmit and/or receive aperturesand an ROI point corresponding to a display pixel may be determined anda table of weighting factors may also be calculated as a function ofsuch total distance. For example, FIG. 4 illustrates examples ofrelationships between S/N weighting factors 40 (vertical axis) that maybe applied to displayed pixels and total path length 42 (horizontalaxis). For example, in some embodiments the S/N weighting factor 40 mayvary linearly 44 as a function of path length. In other embodiments, theS/N weighting factor 40 may vary as a function of path length 42exponentially 46, geometrically or according to any other transferfunction curve such as parabolic functions, normal distributions, lognormal distributions, Gaussian distributions, Kaiser-Besseldistributions, etc. In some embodiments, relationships between totalpath distance and a desired S/N weighting factor may be pre-computedusing one or more transfer functions and stored in a lookup table suchthat, during imaging, an imaging system may look up a weighting factorbased on a determined path length without performing substantialcalculations. In some embodiments, a path length may be pre-calculatedfor each combination of view and ROI point. Such path lengths and/ordesired weighting factors may then be stored in a lookup table which maybe accessed in order to obtain weighting factors during imaging. Inother embodiments, a path length may be estimated or calculated based ona time delay between transmitting a ping and receiving an echo using anassumed speed of sound in the medium being imaged. Thus, in someembodiments, a weighting factor may be dynamically determined based ontime delays determined during beam forming.

Weighting Factors Based on Point Spread

In other embodiments, weighting factors may be used to improve theoverall point spread function (or impulse response) of an imagingsystem. The point spread function (PSF) is well known to those skilledin the art of ultrasound imaging. PSF is the generalized “impulseresponse” of any imaging system, whether acoustic, optical, or otherelectromagnetic radiation. In other words, a figure of merit for anyimaging system is the degree to which a “point” (impulse) in theexamination field is smeared in the component (image layers) and/orfinal images. For the purposes of the present description, point spreadrefers to the degree to which an imaging system ‘smears’ or spreads arepresentation of an object that should appear as a point. The pointspread function for any given ROI point (or represented pixel) may bedetermined as a function of the transmit angle and/or receive anglerelative to a given ROI point. Other factors affecting point spread mayinclude the depth of the ROI point, the degree of coherence, the totalaperture width, individual aperture widths, ultrasound frequency, andother factors.

Ultrasound transducer elements are generally most effective attransmitting and receiving ultrasound signals in a directionperpendicular to the element's surface (i.e., along line 50 in FIG. 5).The sensitivity of a transducer element tends to decrease as the angle θof transmission or reception increases. At some angle θ, image dataobtained from an element may have too little signal strength to beuseful and/or too much noise or point spread to provide valuable imagedata. This is true for both transmit angles and receive angles.(Transmit angles and receive angles may be referred to hereingenerically as “look angles”.) As a result, in some embodiments, it maybe determined that, for a given pixel, data from a particular transmitaperture, a particular receive aperture or a particular view (i.e., aparticular combination of transmit aperture and receive aperture) may beuseful in forming an image of the pixel, but less so than data fromother apertures or views that may contribute to an image of the pixel.In such cases, a fractional weighting factor may be applied to suchlower quality image data in order to decrease its overall contributionto an image or to one or more pixels of an image. In other embodiments,integer weighting factors may also be used.

In some cases, an ideal range of transmit and/or receive angle maydepend on factors such as the material of a transducer array, the sizeor shape of transducer elements, manufacturing methods, element cutshapes, the age of an array, the ultrasound frequency to be transmitted,the voltage or power applied during ultrasound signal transmission, orother factors. In some cases, weighting factors may be applied based onwhether a transmit angle or a receive angle exceeds a particularthreshold value. For example, in some embodiments when ultrasoundsignals are transmitted or received at angles θ greater than a certainthreshold, the signal power may drop dramatically to such a point thatthe signal is overwhelmed by random noise even for relatively smalltotal distances traveled from a transmitter to an ROI point and back toa receiver. In such cases, even though the S/N ratio due to path-lengthattenuation may be very high, the S/N ratio due to the contributions oftransducers at transmit or receive angles in excess of a threshold maybe very low. In some embodiments, the value of such a threshold anglemay be determined by experimentation for a particular transducer type.In other embodiments, the value of a threshold angle may be selectedbased in part on one or more operating parameters such as a transmitfrequency, a transmit power, a transmitted pulse shape, or otherfactors. In some embodiments, some transducers may have a thresholdangle of about 60°, 75°, or 80°. Other transducers may have larger orsmaller quality threshold angles.

In some embodiments, weighting factors may be applied in a binaryfashion based on whether a transmit angle or a receive angle for a givenROI point exceeds a threshold value. For example, in some embodiments, aweighting factor of “1” may be used for all combinations of ROI pointand transducer element for which the angle θ (TX or RX) is less than orequal to a threshold angle, and a weighting factor of “0” may be usedfor any combinations for which the angle θ exceeds the threshold. Inother embodiments, such effects may be counteracted using weightingfactors that are proportional to an angle θ. In other embodiments, acombination of such approaches may be used, such as by using a transferfunction as described in more detail below.

FIG. 6, illustrates two transmit angles relative to pixel ‘A.’ In theillustrated example, a first transmit angle θ_(T1) is shown betweentransmit aperture Lj and point ‘A’. A second transmit angle θ_(T2) isshown between transmit aperture Lj and point ‘A’. As shown, the firsttransmit angle θ_(T1) is substantially larger than the second transmitangle θ_(T2). As a result of this difference in transmit angle, an imageof point ‘A’ formed by pings from transmit aperture Ln will be of higherquality than an image of point ‘A’ formed by pings from transmitaperture Lj since transmit angle θ_(T2) is smaller than transmit angleθ_(T1). Thus in some embodiments using this example, image layers formedby pings from transmit aperture Ln may have a larger weighting factorfor point ‘A’ than image layers formed by pings from aperture Lj. Insome embodiments, the actual and relative values of such weightingfactors may be determined as a function of the relevant transmit anglesbased on a transfer function (examples of which are described below). Insome embodiments, a transmit angle may be measured relative to thecenter of a transmit aperture.

In a similar manner, some embodiments of a multiple aperture imagingsystem may apply weighting factors based on receive angles. FIG. 7illustrates two different receive angles receiving echoes of a reflectorat point ‘A’. A first receive angle θ_(R1) is shown between an elementof the left transducer array 12 and point ‘A,’ and a second receiveangle θ_(R2) is shown between point ‘A’ and an element on the centertransducer array 17. As shown, the first receive angle θ_(R1) issubstantially smaller than the second receive angle θ_(R1). As a resultof this difference in receive angle, an image of point ‘A’ formed byechoes received at the left receive aperture 13 will be of higherquality than an image of point ‘A’ formed by echoes received at thecenter receive aperture 17 since receive angle θ_(R1) is smaller thanreceive angle θ_(R2).

In the illustrated embodiments, each receive aperture has a substantialwidth comprising a plurality of receive elements. Thus for example, areceive angle between point ‘A’ and a receive element at the far leftedge of the center receive aperture 17 will be smaller than a receiveangle between point ‘A’ and a receive element at the far right edge ofthe same center receive aperture 17. Thus, in some embodiments, whendetermining a weighting factor based on a receive angle, the receiveangle may be defined as an angle between the given ROI point and acenter of the receive aperture. In other embodiments, a receive anglemay be defined as a maximum receive angle experienced by a group oftransducer elements in an aperture. Similarly, any of these methods mayalso be used for selecting a transmit angle for transmit aperturescomprising more than one transmitting transducer element.

In other embodiments, weighting factors may be used to correct forcombinations of transmit and receive apertures (i.e., “views”). Theeffect of poorly-contributing views may be mitigated in several ways. Insome cases, a poorly-contributing view may be completely eliminated byignoring echo data received from a particular view, such as by using aweighting factor of zero. For example in some embodiments, for a viewdefined by the transmit aperture Ck and the right receive aperture 15(e.g., as shown in FIG. 3) may be ignored simply by not using echo datareceived by the right receive aperture 15 from a ping transmitted by theCk transmit aperture. Alternatively, if it is determined that all viewsinvolving transmit aperture Ck should be ignored, the system may simplyskip transmit aperture Ck while cycling through transmit apertures.Alternatively, all echo data received from pings transmitted by apertureCk may receive a weighting factor of zero (or nearly zero).

In some embodiments, transmit and/or receive angles may be determined inadvance for each ROI point and such lookup angles may be stored in amemory device. When a zoom level is selected, the imaging system mayidentify ROI points corresponding to image pixels and then identifycorresponding transmit or receive angles. Once transmit or receiveangles are known for a given ROI point in a particular first-level imagelayer, a weighting factor may be determined as a function of one or bothof the transmit and receive angles in order to increase or decrease theeffect of the given view on a final pixel value.

FIG. 8 illustrates examples of transfer functions that may be used fordetermining weighting factors 60 (vertical axis) based on a particularlook angle 62 (i.e., either a transmit angle, a receive angle, or amaximum of the two) for any given combination of view and ROI point. Arelationship between a look angle 62 and a weighting factor 60 mayfollow any of a wide range of patterns. In some embodiments, one or morestepwise functions (e.g., functions with a step increase or decrease inweight at a threshold angle) may be used. For example as shown in curve64 of FIG. 8, weighting factors may be a monotonic function of lookangle in which any pixel-view pair with at least one look angle greaterthan a threshold angle (e.g., about 60°, or π/3 radians, in theillustrated example) may be given a weighting value of zero, while allpixel-view pairs with both look angles less than the threshold angle mayhave a weight of one. In other embodiments, weighting factors 60 may bea linear function 66 of look angle, varying from zero to one as anabsolute look angle 62 increases. In other embodiments, as shown inCurve 65, a transfer function may assign weighting factors of one forall look angles less than or equal to the threshold angle (e.g., 60° inthe illustrated example), and weighting factors may vary linearly fromone to zero for look angles greater than the threshold angle. In furtherembodiments, weighting factors may vary exponentially, geometrically oraccording to any other transfer function curve, including but notlimited to parabolic functions, normal distributions (e.g., as shown bycurve 68), log normal distributions, Gaussian distributions,Kaiser-Bessel distributions, etc. In some embodiments, an ultrasoundimaging control system may be configured to include a plurality ofselectable look-angle-to-weighting-factor transfer functions that may beselected manually by a user or automatically by an imaging system.

In some embodiments, such transfer functions may be implemented withlookup tables. For example, a lookup table may be constructed using achosen transfer function and weighting factors may be calculated forseveral possible discrete values of the relevant input variable (e.g.,TX angle, RX angle, path length, or time delay). Then during imaging, animaging control system may simply determine the input variable quantityand look up a weighting factor value based on the nearest (orinterpolated) result value in the lookup table.

In some other embodiments, instead of determining a weighting factorfrom a transmit and/or receive angle, the point spread for any given ROIpoint corresponding to an image pixel by each view may be predicted bymodeling, or determined empirically using a phantom. In someembodiments, each view may be tested for a given pixel to determinewhether each view improves image quality or makes it worse. Repeatingthis process for each view and for each pixel location, a table ofweighting factors may be assembled. For example, with reference to FIG.3, pixel ‘A’ may be tested by transmitting a first ping from transmitaperture Ln and receiving echoes on the Center receive aperture 17, thentransmitting a second ping from transmit aperture Lj and receivingechoes on the Center receive aperture 17. The results of the two viewsmay then be compared to determine which view provides higher qualitydata for forming pixel A. In some embodiments, such a testing method maybe carried out automatically by modeling the conditions of a givenimaging system and a given multiple aperture probe.

In some embodiments, a lookup table may be assembled to represent theresults of testing each view for each pixel. For example, a lookup tablemay include a weighting value for each unique combination of pixel andview (referred to herein as a pixel-view pair). In some embodiments,such weighting values may be binary such that data from each view eithercontributes to a pixel or is ignored. In other embodiments, weightingvalues may be fractional values, such that each view contributes to apixel in proportion to a weighting factor (e.g., 0% to 100% of dataacquired for each view may contribute to a given pixel's displayedvalue). In some embodiments, such fractional weighting factors may bedetermined by using a transfer function based on any suitable variablerelated to a degree of expected point spread.

One challenge with maintaining a lookup table of weighting values foreach pixel-view pair is that pixel-view relationships change as an imageis “zoomed” or “panned” to display different portions of the insonifiedregions. For example, if the pixel grid 20 is assumed to be a completeinsonified region, and a user “zooms in” on a particular region 30 ofthe insonified region, the information within the zoomed region 30 willbe enlarged to occupy the entire display pixel grid 20. In that case,the ROI points corresponding to displayed pixels will be substantiallydifferent relative to the transmit and receive apertures of the probe ascompared with a fully zoomed-out image. As a result, the contribution ofeach view to each new pixel may be substantially different than in theoriginal “un-zoomed” image. For example, if a selected zoomed region issmall enough and sufficiently far from the probe, every value of aweighting mask may simply be one.

In some embodiments, this challenge may be addressed by computing andstoring pixel-view weighting values for a plurality of discrete zoomlevels or pan locations. In such embodiments, separate weighting valuetables would be needed for each pre-computed zoom level. In someembodiments, weighting values for zoom levels in between pre-computedzoom levels may be interpolated, or the nearest table may be used. Inother embodiments, weighting factors may be determined on-the-fly usinga transfer function to identify a weighting factor based on a measurableor detectable variable quantity such as transmit angle or receive angleas described above.

Combining Weighting Factors

In some embodiments, S/N weighting factors may be combined with pointspread weighting factors, look angle threshold weighting factors,transmit frequency weighting factors and/or weighting factors of anyother type. In some embodiments, multiple distinct weighting factors maybe combined by simple arithmetic averaging. In other embodiments,weighted arithmetic averaging may be used to increase or decrease therelative impact of one type of weighting factor relative to one or moreother type. As discussed above, the intensity magnitude of any givenpixel I_(p) may be obtained by: I_(p)=Σw*I_(v). Where w is a weightingfactor and I_(v) is the intensity obtained by a particular view (v).Thus, in some embodiments, the weighting factor ‘w’ may be determinedby: w=Aw₁+Bw₂, where A and B are weighted average coefficients, and w₁and w₂ are different types of weighting factors. For example, if thecoefficients A & B are both 0.5, then equal weight will be given to theweighting factors w₁ & w₂. On the other hand, for example, if A is 0.75and B is 0.25, w₁ will be given three times the weight of w₂. In furtherembodiments, weighting factors of multiple types may be combined by morecomplex normalizing algorithms which may be based on factors such as thetype of weighting factor, the location of pixels, or other factors. Anyother combining or normalizing algorithms may also be used. Any of theabove approaches to combining weighting factors may be applied topixel-specific tables of weighting factors, array-specific tables, orscalar weighting factors.

In other embodiments, a common weighting factor may be applied to all orany portion of one or more first-level image layers. For example, if itis determined that a given transmit aperture is providing low qualityimaging data, all pixels may have a weighting factor of zero for allviews or image layers containing data obtained from that transmitaperture.

In some embodiments, an ultrasound imaging system may include manual orautomatic controls configured to allow for manual or automaticadjustment of weighting factors by aperture. For example, in someembodiments, an ultrasound imaging system may include an array ofsliders (or other physical or virtual control devices), with one sliderfor each transmit aperture. Adjusting such sliders may increase ordecrease a weighting factor for a particular transmit aperture for allviews. Similarly, such controls may be provided for each receiveaperture. For example, while imaging a region of interest, a user mayadjust a given aperture control in order to increase or decrease acontribution of that aperture to the displayed image until the userdetermines that an adequate or optimal image has been obtained.

In further embodiments, a multiple aperture imaging system may beconfigured to automatically increase or decrease weighting factors forindividual transmit and/or receive apertures or elements until a desiredimage quality metric is optimized. In some embodiments, image qualitymetrics that may be optimized by adjusting weighting factors may includeimage sharpness, contrast, dynamic range, point spread, or othermetrics. In some embodiments, optimization may comprise identifying agroup of weighting factors that maximizes the selected image qualityvariable. In other embodiments, optimization may comprise identifying agroup of weighting factors that minimizes the selected image qualityvariable. In still other embodiments, optimization may comprisemaximizing or minimizing a group of image quality variables whileremaining within additional constraints. In various embodiments, whenapplying weighting factors to a collection of transducer elements (orapertures), a single scalar may be stored and applied to all relevantpixels rather than storing and applying a table of weighting factors.

Transmitting Multiple Ping Frequencies

In other embodiments, weighting factors may be developed to normalizethe effect of other factors that may otherwise cause some transmit orreceive elements to have a distorting effect on a final image. Forexample, in some cases an ultrasound probe may include one or moretransmit transducer elements configured to transmit ultrasound signalswith different power levels, different fundamental frequencies,different pulse shapes and/or different pulse lengths as compared withother transmit (and/or receive) transducer elements. In such cases, itmay be desirable to increase or decrease the relative contribution ofechoes from such transducer elements to a final combined image. As withthe previous embodiments, such transmit and/or receive elements may beweighted individually or in groups according to manual controls orautomatic algorithms. Such additional weighting factors may also becombined with other types of weighting factors as described above.

For example, as is generally understood by those skilled in the art,high frequency pulses can produce higher quality images but cannotpenetrate as far into the body. On the other hand, lower frequencypulses may penetrate deeply, and may therefore produce higher qualityimages of deeper tissue, but will tend to produce lower quality imagesof shallow tissue compared with higher frequency pulses. Thus, in someembodiments, an ultrasound imaging system may be configured to transmitand receive low-frequency pulses to image deep regions of an imageframe. The same system may also be configured to transmit and receivehigh frequency pulses to image relatively shallow regions of the imageframe. In such a system, low-frequency image layers may be combined withthe high-frequency image layers to improve the quality of all regions ofa combined image. In some embodiments, weighting factors may be used toincrease the contribution of deep regions of the low frequency imagelayer, to increase the contribution of shallow regions of the highfrequency image layer, and to smooth a transition between the deep andshallow regions.

In similar ways, further embodiments may be configured to adjust atransmit signal (e.g., by adjusting a transmit pulse frequency, shape,time length, etc.) to optimize one or more selected regions of an imageframe, and the resulting regionally-optimized image layer may becombined with other image layers that may be optimized in differentregions. In other examples, regionally-optimized image layers may becombined with image layers that are not regionally optimized. In someembodiments, weighting factors may be used to smooth transitions or tootherwise improve the quality of a combined image. For example,weighting factors may be used to increase the contribution of pixelswithin an optimized region of an image while decreasing the contributionof non-optimized image regions. In further embodiments, weightingfactors may be used to smooth a transition between an optimized regionof one image layer and adjacent regions of other image layers.

In some embodiments, after applying any weighting masks and/or aftercombining image layers, a normalizing step may be applied in order tocause all regions of a final image (e.g., a third-level image) to have aconsistent average intensity. For example, without normalizing pixelintensities, lateral and/or corner regions of a final image may besubstantially less bright than a central region of the image due to theapplication of weighting factors. Thus, in order to provide a moreconsistent image, intensity levels for all pixels in an entire image maybe normalized to fall within a desired range. In some embodiments, suchnormalization may be achieved by techniques similar to those employed bya lateral gain control of a standard ultrasound imaging system, whichincreases brightness of otherwise relatively “dim” pixels at lateraledges of an image.

Weighting Factors to Avoid Obstacles

FIG. 9 illustrates an ultrasound imaging scenario in which some transmittransducers of a multiple aperture probe 11 are partially or completelyobscured by an obstacle 70. In some embodiments, the obstacle 70 may bea rib or other bone in a human or animal subject. In other embodiments,an obstacle may be a material with a very high or very low inherentspeed-of-sound relative to surrounding material being imaged. Forexample, bone has a high inherent speed of sound relative to tissue, andan air-filled organ such as a lung will typically have a much lowerspeed-of-sound than surrounding tissues. Alternatively, any otherstructure that interferes with a desired image may be interpreted as anobstacle. Multiple aperture ultrasound imaging systems may produce animage using an un-modified multiple aperture imaging technique in thescenario of FIG. 9, but the obstacle will typically cause a bright“halo” effect in the near field and a shadow beyond the obstacle, sincea hard obstacle will echo substantially all of the ultrasound energytransmitted toward it. By contrast, with single-aperture (and especiallyphased-array) imaging systems, any obstacle will tend to entirelyeclipse a substantial section of image data beyond the obstacle,resulting in a null set of image data for the eclipsed region. Thus, insome embodiments, it may be desirable to ignore or reduce the effect oftransmit pings from transmit apertures that are entirely or partiallyobscured by an obstacle. In some embodiments, a degree of obstacleblockage may be determined and then a weighting factor may be selectedas a function of the degree of blockage.

As an example, when trying to image tissue behind obstructions such asribs, an imaging system using ping technology can be configured to makeuse of signals returning from the deep tissues and can, to a largeextent, ignore the signals that were blocked. However, when thebeamformer does incorporate the signals that are received after ablocked transmission, channel noise is typically added into the image.For this reason, it is desirable to detect blocked transmitters and notuse the corresponding received signals which are mostly noise.

In some embodiments, a trial and error process may be used to identifytransmit apertures that are not obscured by obstacles. In someembodiments, a given transmit aperture may be tested by transmitting aping from that transmit aperture and listening for return echoes at oneor more receive apertures. Received echoes with magnitudes greater thansome specific threshold value, occurring after relatively long timedelays, may indicate that the ultrasound signals are penetrating to asubstantial depth within the region of interest, and the test transmitaperture therefore may be qualified as being essentially free ofblocking obstacles. In some embodiments, transmit apertures that returnno deep echoes at all may be interpreted as being entirely blocked by anobstacle. However, this is an imperfect method because a lack of deepechoes may also indicate very anatomically and acoustically uniformmaterial or some other material that simply does not contain anysignificant reflectors at the test depth.

In other embodiments, it may be preferable to directly identify blockedtransmit apertures rather than attempting to identify unblocked transmitapertures. In some embodiments, a transmit aperture may be tested forblockage by transmitting a ping and evaluating return echoes in the nearfield using, for example, a temporal gating or windowing mechanism toeliminate potential false positive results due to anticipated strongechoes occurring at or just above the skin surface. This may beimportant to preclude strong echoes likely to be received from thetransducer-to-lens interface, the lens-to-gel interface, and thegel-to-skin interface from being incorrectly interpreted as blockingobstacles. Accordingly, the test depth can be restricted by establishinga temporal “starting point” for return echoes to be examined, andsamples arriving prior to the start of the window are likely interfaceechoes that can safely be ignored. Similarly, a temporal “ending point”for strong return echoes can be established to rule out deep structuresbeneath the region of interest from being classified as blockingobstacles; any such echoes detected may also be ignored. If the echoesreceived from a test ping are substantially strong ones with time delaysoccurring within the appropriately defined gate or window, a hardblocking obstacle is likely present between the probe and the region ofinterest, and the test transmit aperture may be classified as beingpotentially blocked by this obstacle. If a test ping does not return anysubstantially strong echoes within the appropriately defined test depth,that transmit aperture may be assumed to be clear of blocking obstacles.In some embodiments, the degree or extent of blockage may be evaluatedby analyzing the patterns of strong shallow echoes and deeper echoesoccurring at multiple receive apertures.

In some embodiments, a test depth at which an obstacle is anticipatedcan differ based on variability of the body or object under inspection.For example, substantial variation in the thickness of a fat layer overthe ribs from one patient to another may cause significant variation inthe test depth at which the imaging system may evaluate echoes toidentify obstacles. In some embodiments, a variable window/depth controlmay be provided to allow manual or automatic adjustment of an evaluationdepth for identifying obstacles. For example in some embodiments, atest-depth control may be set to evaluate echoes at depths between 1 mmand 1 cm (or more) below the probe. Such a control may be adjusted toevaluate echoes at various depths in order to locate a range of depthsreturning strong echoes indicating the presence of an obstacle. In someembodiments, the width of such a test window may be held constant whileseeking echoes at various depths. In some embodiments, a range of depthat which the system may search for obstacles may be determinedautomatically. In such embodiments, a probe may be placed over a known(or expected) obstacle, and a process may be initiated in which animaging control system transmits pings and “listens” for strong echoeswithin a particular depth range.

In some embodiments, weighting factors may be applied to transmitapertures based on a degree to which each transmit aperture is obscuredby an obstacle. For example, a transmit aperture that is entirelyobscured by an obstacle may receive a weighting factor of zero (or avalue substantially near zero). A transmit aperture that is entirelyclear (i.e., not obscured by any obstacles at all) may have a weightingfactor of one (or substantially near one). In some embodiments,partially or entirely blocked transmit apertures may have weightingfactors applied with respect to all receive apertures. In otherembodiments, partially or entirely blocked transmit apertures may haveweighting factors applied with respect to only some receive apertures.In still other embodiments, weighting factors may be applied based onROI point location, such as applying different weights for shallow ROIregions above a blocking obstacle relative to regions below (i.e.,blocked by) the obstacle. For example, ROI points above an obstacle mayreceive weighting factors of about one, while ROI points below theobstacle may receive a weight of about zero.

In some embodiments, transmit apertures that are only partially obscuredmay have weighting factors between zero and one in proportion to adegree of blockage. With reference to FIG. 9, transmit aperture Lj maybe interpreted as entirely blocked by the obstacle 70, since nearly allof the energy transmit by aperture Lj will be reflected by the obstacle70. However, aperture Lk is only partially blocked by the obstacle 70and some substantial amount of energy transmitted by aperture Lk willpass through the region of interest and will be reflected to at leastthe center 17 and right 15 receive apertures. Transmit aperture Ln ispartially blocked by the obstacle 70, since some ultrasound energy willstill pass through the region of interest around the obstacle 70. Insome embodiments, weighting factors may be applied to transmit aperturesalone in order to improve image quality in the area of a detectedobstacle. For example, in the situation illustrated in FIG. 9, aweighting factor for all image pixels may resemble the following:

TABLE 1 Weighting Factors for a Blocked TX Aperture RX Left TX Ln 0.3 TXLj 0 TX Lk .5 TX Cn 1 TX Cj 1 TX Ck 1 TX Rn 1 TX Rj 1 TX Rk 1

In other embodiments, both individual transmit elements and individualreceive elements may be weighted to address detected obstacles. Forexample, in the situation illustrated in FIG. 9 (and assuming that theelements Ln, Lj, Lk, Cn, Cj, Ck, Rn, Rj, and Rk may also be used asreceive elements), a weighting factor table for all image pixels mayresemble the following:

TABLE 2 TX and RX Weighting Factors for a Blocked TX Aperture RX RX RXRX RX RX RX RX RX Ln Lj Lk Cn Cj Ck Rn Rj Rk TX Ln 0.3 0.0 0.4 0.5 0.50.5 0.5 0.5 0.5 TX Lj 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 TX Lk 0.4 0.00.5 0.7 0.7 0.7 0.7 0.7 0.7 TX Cn 0.5 0.0 0.7 1 1 1 1 1 1 TX Cj 0.5 0.00.7 1 1 1 1 1 1 TX Ck 0.5 0.0 0.7 1 1 1 1 1 1 TX Rn 0.5 0.0 0.7 1 1 1 11 1 TX Rj 0.5 0.0 0.7 1 1 1 1 1 1 TX Rk 0.5 0.0 0.7 1 1 1 1 1 1

In some embodiments, a general smoothing function may be applied basedon expected geometry of a particular type of obstacle. For example, ifit is known that expected obstacles are ribs, certain assumptions can bemade regarding the geometry of detected obstacles, such as a range ofanticipated rib size, spacing between ribs, ranges of depth at whichribs may be found, etc. In some embodiments, such information may beused to correct measurement errors. For example, an indication that anostensibly un-blocked transmit aperture is positioned in between two ormore closely spaced blocked apertures could be safely interpreted as anerror. As a result, the ostensibly unblocked transmit aperture may beignored and may be treated as “blocked.” Similarly, if the spacingbetween ribs is assumed to fall within a known range, an indication thata blocked transmit aperture is positioned between two or more closelyspaced clear apertures may also be interpreted as an error.

In other embodiments, known or assumed information about the geometry ofobstacles within a region of interest may be used to smooth a transitionbetween “blocked” and “un-blocked” transmit apertures. For example,during B-mode imaging, transmit apertures that are positioned at an edgeof an obstacle can in some cases experience refraction and/ordiffraction of the ultrasound signal. Thus, in some embodiments,transmit apertures adjacent to an edge of a detected obstacle may beassigned weighting factors that increase from zero (for blockedapertures) to one (for entirely clear apertures) in steps, therebyminimizing the effect of transmit apertures that may still be adjacentto (and/or partially blocked by) the obstacle. In other embodiments,transmit apertures that are only partially blocked, or that aredetermined to be too close to detected obstacles may be ignored.

In some embodiments, in addition to improving the quality of B-modeultrasound images, the identification of “clear” transmit apertures maybe beneficial for performing Doppler imaging or Elastography with amultiple aperture probe. Some embodiments of Doppler imaging andElastography utilize a single transmit aperture for obtaining multipleimages at extremely high frame rates (e.g., hundreds or thousands offrames per second). In such embodiments, the above methods may be usedto identify one or more suitable transmit apertures that are well clearof any detected obstacles. For example, if two adjacent obstacles areidentified (such as two adjacent ribs), an imaging control system mayselect a transmit aperture that is in between the two obstacles. In someembodiments, such a selected transmit aperture may be equidistant fromboth detected obstacles.

Any of the foregoing embodiments may be used in combination with amultiple aperture imaging probe of any desired construction. Examples ofmultiple aperture ultrasound imaging probes are provided in Applicant'sprior patent applications, including the following US PatentApplications: U.S. patent application Ser. No. 11/865,501, filed Oct. 1,2007 and titled “Method And Apparatus To Produce Ultrasonic Images UsingMultiple Apertures,” now U.S. Pat. No. 8,007,439; U.S. patentapplication Ser. No. 12/760,375, filed Apr. 14, 2010, published as2010/0262013 and titled “Universal Multiple Aperture Medical UltrasoundProbe”; U.S. patent application Ser. No. 12/760,327, filed Apr. 14,2010, published as 2010/0268503 and titled “Multiple Aperture UltrasoundArray Alignment Fixture”; U.S. patent application Ser. No. 13/279,110,filed Oct. 21, 2011, published as 2012/0057428 and titled “Calibrationof Ultrasound Probes”; U.S. patent application Ser. No. 13/272,098,filed Oct. 12, 2011, published 2012/0095347 and titled “MultipleAperture Probe Internal Apparatus and Cable Assemblies”; U.S. patentapplication Ser. No. 13/272,105, filed Oct. 12, 2011, published as2012/0095343 and titled “Concave Ultrasound Transducers and 3D Arrays”;U.S. patent application Ser. No. 13/029,907, filed Feb. 17, 2011,published as 2011/0201933 and titled “Point Source Transmission AndSpeed-Of-Sound Correction Using Multi-Aperture Ultrasound Imaging”. Theentire contents of each of these patents and patent applications isincorporated herein by reference.

Embodiments of the systems and methods described above may also bebeneficially applied to multiple aperture ultrasound imaging systemsutilizing focused phased array transmit pulses rather than point sourcetransmit pulses (pings). Similarly, embodiments of the systems andmethods described above may also be beneficially applied tosingle-aperture imaging systems using multiple sub-apertures for pingtransmission. In still further embodiments, the methods described abovemay also be applied to conventional ultrasound systems using phasedarray-transmissions from a single-aperture probe.

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Various modifications to the above embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, it is intended that the scope ofthe present invention herein disclosed should not be limited by theparticular disclosed embodiments described above, but should bedetermined only by a fair reading of the claims that follow.

In particular, materials and manufacturing techniques may be employed aswithin the level of those with skill in the relevant art. Furthermore,reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “and,” “said,” and “the”include plural referents unless the context clearly dictates otherwise.As used herein, unless explicitly stated otherwise, the term “or” isinclusive of all presented alternatives, and means essentially the sameas the commonly used phrase “and/or.” It is further noted that theclaims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

What is claimed is:
 1. A method of identifying transmit elements notblocked by an obstacle, the method comprising: transmitting an unfocusedfirst circular wave front ultrasound signal from a first transmitaperture and receiving echoes of the first circular wave frontultrasound signal at a first receive aperture; determining if deep echoreturns from within the region of interest are received by identifyingif a time delay associated with the received echoes exceeds a thresholdvalue; and identifying the first transmit aperture as being clear of anobstacle if deep echo returns are received.
 2. The method of claim 1,further comprising applying a weighting factor of substantially near 1to the first transmit aperture.
 3. The method of claim 1, furthercomprising transmitting an unfocused second circular wave frontultrasound signal from a second transmit aperture and receiving echoesof the first circular wave front ultrasound signal at the first receiveaperture or a second receive aperture.
 4. The method of claim 3, furthercomprising identifying the second transmit aperture as being obscured byan obstacle if deep echo returns are not received.
 5. The method ofclaim 4, further comprising applying a weighting factor of less than 1to the second transmit aperture.
 6. A method of identifying transducerelements blocked by an obstacle, the method comprising: transmitting anunfocused first circular wave front ultrasound signal from a firsttransmit aperture and receiving echoes of the first circular wave frontultrasound signal at a first receive aperture; determining whetherstrong shallow echo returns are received by identifying a plurality ofecho returns with intensity values greater than a threshold intensityand with time delays less than a threshold time delay; and identifyingthe first transmit aperture as being blocked by an obstacle if strongshallow echo returns are received.
 7. The method of claim 6, furthercomprising applying a weighting factor of substantially near 1 to thefirst transmit aperture.
 8. The method of claim 6, further comprisingtransmitting an unfocused second circular wave front ultrasound signalfrom a second transmit aperture and receiving echoes of the firstcircular wave front ultrasound signal at the first receive aperture or asecond receive aperture.
 9. The method of claim 8, further comprisingidentifying the second transmit aperture as being obscured by anobstacle if strong shallow echo returns are not received.
 10. The methodof claim 9, further comprising applying a weighting factor of less than1 to the second transmit aperture.
 11. An ultrasound imaging systemcomprising: an ultrasound transmitter configured to transmit unfocusedultrasound signals into a region of interest; an ultrasound receiverconfigured to receive ultrasound echo signals returned by reflectors inthe region of interest; a beamforming module configured to determinepositions of the reflectors within the region of interest for displayingimages of the reflectors on a display; first user-adjustable controlsconfigured to select a designated aperture from a plurality of transmitapertures and receive apertures of the ultrasound transmitter andultrasound receiver; and second user-adjustable controls configured toincrease or decrease a speed-of-sound value used by the beamformingmodule to determine the positions of reflectors detected with thedesignated aperture.
 12. The system of claim 11, wherein the designatedaperture is a transmit aperture.
 13. The system of claim 11, wherein thedesignated aperture is a receive aperture.